Seismic instruments: article

Error evaluation of the VES inverse problem solution for precision investigations of time variations in geoelectric section with strong seasonal effect
A.A. Bobachev A.V. Deshcherevskii A.Ya. Sidorin
Schmidt Institute of Physics of the Earth, Russian Academy of Sciences
Journal: Seismic instruments
Tome: 58
Number: 4
Year: 2022
Pages: 41-61
UDK: 550.837.311
DOI: 10.21455/si2022.4-3
Keywords: geoelectric monitoring, precision soundings, VES, data inversion error, seasonal variations, flicker noise
Аnnotation: As part of studies on the search for earthquake precursors, the authors conducted an experiment on long-term precision monitoring of variations in the resistivity of the Earth's crust in a highly seismic region of Tajikistan. The primary data of this experiment can be considered as a VES profile of a special type, in which, instead of a linear coordinate, the sounding date changes from picket to picket. When processing precision monitoring data, it is necessary to solve the inverse VES problem with the highest possible accuracy. VES curve inversion pro-grams commonly used in electrical exploration do not allow this. The authors have previously developed a special method for regularizing the residual functional, which suppresses the effect of resistivity buildup, due to which the error in reconstructing the electrical resistivity of rocks for profiles with a strong seasonal variation in resistivity is reduced by an order of magnitude. However, in some cases, the regularized algorithm strongly bias-es estimate of the amplitude of the seasonal resistivity variation in the lower layers of the section. In this paper, the work of the proposed algorithm is tested in detail for nine model profiles that simulate a real geoelectric section. The considered profiles differed in the characteristics of the seasonal course of resistivity in the lower layers of the section (the phase and amplitude of seasonal effects varied). It is shown that the buildup of resistivity is effectively suppressed in all cases. For each model profile, the error in solving the inverse problem is evaluated. The effect of estimate shifting in the amplitude of the seasonal variation is studied. It is shown that in most cas-es the analysis of the solution makes it possible to reveal the presence of such distortions and qualitatively assess their character. It is also shown that for those profile options that are supposedly closest to the experimental profile, the bias of the estimates is minimal. For all profiles, the ratio of the average and maximum errors in the calculation of resistivity in different layers to the discrepancy in the solution of the inverse problem was evaluated. This makes it possible to evaluate the actual error of the reconstructed resistivity values knowing only the selection discrepancy. The paper also studied the possible effect of increasing the accuracy of solving the in-verse problem in the case of preliminary decomposition of the apparent resistivity curves into seasonal and flicker noise components. It is shown that for small selection residuals, the results change insignificantly. Accord-ing to the results obtained, the error in reconstructing the aperiodic (flicker noise) component of resistivity varia-tions in the lower layers of the considered section can be decreased to 0.4 %. The accuracy of reconstructing the seasonal component of resistivity variations depends on the amplitude and phase of sea-sonal effects in the model profile. For the considered profiles, the error varies from 1 to 2 %.